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Full text of "Handbook Of Chemical Engineering - I"

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POWER GENERATION AND TRANSMISSION
69
	Full-load	Three-fourths load	One-half load	One-fourth load
f A •"• .........	1 0	1 02-1 15	1 07-1 25	1 40-1 90
Relative fuel rates \ B .........	1.0	1 05	1 16	
C ..........	1.0	1.06	1.17	
				
On account of the high pressures used, Diesel (and semi-Diesel) engines cannot be built with very large cylinders. They are heavy and costly. For large powers, a large number of cylinders is necessary. This again implies great bulk, weight and cost. The air compression plant is large and complicated.
Place of the Oil Power Plant.—While oil engines are expensive, and first cost varies in the same way as efficiency, the engine constitutes almost the whole plant, because (almost alone among heat engines) it uses raw fuel.
Its high economy goes far to offset the usually high cost of fuel per British thermal unit. Maintenance costs are high, and miscellaneous operating costs not always low Reliability has not been thoroughly demonstrated. Diesel and semi-Diesel engines are complicated and employ high pressures. Low-pressure oil engines are somewhat uncertain as to ignition. The assured field at present seems to be for small isolated powers, although large installations and some marine plants have been successfully operated.
The expression given for horsepower on page 63 applies both to hot-cap and Diesel engines. Losses due to jackets and exhaust are about as for gas engines. These engines are always water-cooled. Engines of low pressure type cost two to three times as much as steam turbines: high-pressure and Diesel engines are about twice as costly per horsepower as low-pressure. There has been little attempt to use asphaltum base (Western) oils in internal-combustion engines.
WATERPOWER
Water Wheels.—The older types of wheel give maximum efficiencies up to 70 to 90 per cent when used under the particular conditions for which they are suitable: i.e., for heads up to 70 ft. and powers not exceeding 75 hp. Important wheels are turbines. These are of the tangential (impulse) or reaction type. Tangential turbines use the velocity of water to produce power, the head being wholly converted to velocity in open-end nozzles. They run at low peripheral speeds for given heads and are therefore the type best adapted to high heads (300 to 3,000 ft.) and small quantities of water. Their construction is simple. The runners carry double-lobed buckets toward which the jets are directed and the stream is deflected about 90° on both sides of the "splitter."
Reaction turbines utilize the pressure, velocity and weight of water. The Francis type universally used in this country receives the water through stationary guide vanes which carry it radially inward and partially convert the head into velocity. The discharge is parallel with and close to the shaft. Such turbines are adapted for heads of 5 to 600 ft. They may be vertical or horizontal: the former are better adapted for direct-connection where water levels fluctuate, but require thrust bearings. For heads of 5 to 50 ft., the turbines are often set in an open flume. For higher heads, they are encased, the spiral form of casing being best. Some vertical low-head turbines